The present invention generally relates to methods and apparatus for controlling multi-phase electric machines and, more particularly, protecting such machines from damage that may result from non-symmetrical short circuit failures.
In many electric machine applications it is desirable to employ a synchronous machine with a permanent magnet rotor. Such a machine may be referred to as a permanent magnet machine (PMM). While a PMM may have advantageous features, it has a disadvantageous feature that has heretofore limited its use in some applications.
Rotor flux in a PMM is fixed and cannot be controlled or disengaged when a short circuit occurs. Unlike other electric machines where excitation of the rotor flux can be controlled and even shutdown quickly, a PMM continues to generate electromagnetic flux until the rotor stops. Therefore, a PMM may present a hazard in some applications in which rotor speed may not be quickly reduced in the event of a short-circuit failure.
Some specialized PMM's have been designed so that they may tolerate some short-circuit failures with continuing rotation of a rotor. For example, a high-reactance permanent magnet machine (HRPMM) may internally limit the phase current magnitude, should it become shorted. The high reactance nature of this type of machine sustains short circuit currents either indefinitely, within the thermal limits of the system, or until the rotor speed can be reduced to zero. This fault protection method may accommodate a so called, bolted short or symmetrical short, i.e. all phases of the machine connected together electrically. However, in some instances a short circuit may develop in a single phase, between two phases or as an intermittent fault rather than as a symmetrical short. Such a failure may be referred to as a non-symmetrical short circuit failure (NSSCF). A NSSCF may create negative sequence flux that may produce overheating of the rotor assembly.
It may be seen that there is a need to provide a PMM which may tolerate a NSSCF with continued rotor rotation. There is a need for such a PMM which may tolerate occurrence of a NSSCF without experiencing destructive overheating of the rotor assembly.